Stabilization of styrene



Patented June 1, 1943 2,320,859 s'ranmzarron or srrnm Stanley GeorgeFoord, London, to International Standard tion, New York, N. Y.

No Drawint. Original plication November 14, 19 In Great Britain Ailril1,

Claims.

This invention relates to a process of treating styrene by the use 01'inhibitors of polymerisation that are only sparingly soluble in styrene(i. e., to the extent of less than 0.1%).

Such inhibitors appear to act in the inhibition of polymerisation byslowly combining with the styrene. During such combination onpolymerisation takes place and the period elapsing before the start ofpolymerization is herein termed the induction period. Thus an extendedinduction period can be produced by adding the required amount of.sparingly soluble inhibitor to the styrene. Some of the inhibitordissolves in the styrene, i. e., enough to form a saturated solution. Asthis inhibitor is used up by reacting with the styrene more goes intosolution. Whatever the theoretical explanation, the undissolved excessof inhibitor is gradually used up during the induction period. Suchsparingly soluble inhibitors are catechol, hexamine (i. e.,hexamethylene tetramine), hydroquinone, hydroxylamine hydrochloride,resorcinol, 1.5 dinitroanthraquinone, and an inorganic salt of methylp.aminophenol such as the sulphate (metol).

I-lydroquinone has previously been suggested as a stabilizer of styreneagainst polymerisation, but the conclusion was reached that in a givenperiod of time a lessened degree 01' polymerisation would take place inthe presence of hydroquinone than in its absence. The other substancesmentioned above have, however, not previously been suggested asstabilizers of styrene for an extended period. Moreover, the manner inwhich these sparingly soluble substances mentioned above act ininhibiting polymerisation of styrene is my discovery.

When using an inhibitor of polymerisation oi styrene, it is necessary toremove the inhibiting effect before the polymerisation. with knowninhibitors such as benzoquinone and hydroquinone it has previously beenthought necessary to remove excess of inhibitor from the styrene by somesuch process as washing out with caustic soda or distillation underreduced pressure.

At an elevated temperature a sparingly soluble inhibitor when present inexcess produces a period during which no polymerisation takes place andthereafter may produce a period during which the polymerisation isconsiderably retarded. If therefore styrene be heated with undissolvedexcess of inhibitor present, in an attempt to polymerise the styrene,the inhibitor in solution is used up in combining with styrene and thusfirst inhibiting andthen retarding poly-- merisation, and as it is usedup more inhibitor England, assignor Electric Corporaapplication December14, 1942, Serial No. 204,975.

Divided and this ap- 40, Serial No. 365,658. 1938 goes into solution andwhen in solution continues the retarding of polymerisation until all theinhibitor both free and dissolved is used up.

It will be seen therefore that the inhibitor must be removed beforepolymerisation is attempted, if this is to be completed in anyreasonable time. I have discovered however that when a sparingly solubleinhibitor is used it is only necessary to remove the undissolvedinhibitor. I have discovered that the extent to which the time ofheating required to eilect polymerisation is prolonged by the amount ofinhibitor remaining in solution, i. e., the amount required to produce asaturated solution, is inconsiderable at the temperatures normally usedfor polymerisation, e. g., C.

According to the present invention a process for treating styrenecomprises adding to the styrene an amount of a sparingly solubleinhibitor oi. polymerisation sufllcient to maintain a saturated solutionthereof in the styrene over the a period of time during whichpolymerisation oi the styrene is to be inhibited and then filtering oflundissolved inhibitor prior to polymerisation of the styrene.

The following table shows the induction periods of styrene at 60 C.caused by various sparingly soluble inhibitors when added in thepercentage by weight stated in the second column of thetable:

At the completion of the induction period the styrene commences topolymerise at a rate which rapidly increases to a maximum, and thencontinues steadily, the rate of polymerisation being The inductionperiod for each of the above substances is given at 60 C. instead or atroom temperature because of the length oi time that would be necessaryto verify the length of the induction period at room temperature, sincethis is very long, being related to the induction period at highertemperatures by an exponential law.

I have iound, however. that with these sparingly soluble inhibitors,filtration is sufficient to remove all inhibitor save such a smallquantity that the increase in induction period at polymerisationtemperature, e. g., 120 0., due to the presence of that quantity, isinappreciable.

A good inhibitor for this p p se is hydroquinone. I have found thathydroquinone dissolves in styrene at room temperatures only to theextent oi 0.001%. Consequently, in order to polymerise styrene to whichhas been added a large amount of hydroquinone, the undissolvedhydroquinone is filtered ofl, leaving styrene with only 0.001% ofdissolved hydroquinone. The induction period at 120 C. of this materialis not more than 10 minutes, i. e., inappreciably greater than that ofpure styrene.

Some oi the substances mentioned are, compared with hydroquinone, muchmore soluble, but still must be classed as sparingly soluble, i. e., tothe extent of less than 0.1%. It has been mentioned above that catecholnot only acts as an inhibitor o1 polymerisation but also slows down therate oi. polymerisation at the end or the induction period. This lattereffect is only appreciable, however, in the presence of so much catecholthat some of it is originally undissolved, but after filtration theeffect 01 the amount left in solution is inappreciable.

The present application is a division of my U. S. Patent 2,225,471,issued December 17, 1940, and entitled Stabilization of styrene.

What is claimed is:

1. A process of inhibiting styrene against polymerisation whichcomprises adding 1.5 dinitroanthraquinone to the styrene.

2. Styrene inhibited against polymerisation by the addition of 1.5dinitroanthraquinone.

3. A process of inhibiting styrene against polymerisation whichcomprises adding 1.5 dinitroanthraquinone to thestyrene in a proportionnot more than 0.5% of the styrene.

4. A styrene composition comprising styrene together with 1.5dinitroanthraquinone, the proportion of 1.5 dinitroanthraquinone beingnot more than 0.5% oi! the styrene.

5. A styrene composition comprising a saturated solution of 1.5dinitroanthraquinone in styrene.

STANLEY-GEORGE FOORD.

